The present invention relates to a method for making cyclic oligomeric aromatic polycarbonates from monochloroformate oligomeric aromatic polycarbonate compositions.
Prior to the present invention, as shown by Brunelle and Shannon, U.S. Pat. Nos. 4,644,053 and 4,638,077, aromatic bischloroformates were prepared which were cyclized via a hydrolysis/condensation mechanism. Although valuable results are achieved utilizing the Brunelle and Shannon procedures, it has been found that during cyclization of the bischloroformate oligomers, about half of the chloroformate end groups are hydrolyzed to the corresponding aromatic hydroxyl endgroup before condensation takes place, i.e., the cyclization is hydrolysis limited. These chloroformate endgroups represent COCl.sub.2 that is consumed during oligomer synthesis, but must be hydrolyzed to allow condensation to occur. Because of safety and environmental considerations, it is desirable to minimize the net COCl.sub.2 requirements for cyclic carbonate synthesis. Hydrolysis of the bischloroformate consisting essentially of condensed carbonate units and included within the formula, ##STR1## where R is a divalent C.sub.(6-30) aromatic organic radical, and n is an integer having a value of 0 to 40, requires 1-3 mol NaOH per mol of chloroformate end group hydrolyzed and generates by-products which include carbon dioxide, sodium carbonate, sodium bicarbonate and sodium chloride. These by-products require extra water to prevent salt precipitation. On the other hand, condensation of aromatic hydroxyl end groups with chloroformate end groups requires only 1 mol of base, per mol of chloroformate end group.
Further, since the slow step in the hydrolysis/condensation of bischloroformates during cyclization is hydrolysis of the chloroformate end group, chloroformates and amine/chloroformate complexes (acylammonium salts) are much more abundant in the mixture than aromatic hydroxyls. The acylammonium salt-terminated oligomers formed during the reactions between organic amine and the chloroformate end groups of the bischloroformate oligomer of formula (1), are interfacially active and can cause emulsification during cyclization. Experience has shown that during the period when the cyclization mixture is emulsified, it is often difficult to determine an accurate pH reading with a pH electrode. Inaccurate pH measurements can result in the addition of excessive alkali metal hydroxide to the cyclization mixture. In particular situations, product degradation manifested by ring opening to linear polymer and unreacted bisphenol A can occur. On the other hand, if insufficient alkali metal hydroxide is added to the cyclization mixture, undesirable levels of linear polycarbonate can be formed.
In copending application serial number 519,980, a method is shown for making oligomeric bisphenol monochloroformate polycarbonate oligomers. The bisphenol monochloroformate oligomers are comprised essentially of condensed carbonate units and are included within the formula, ##STR2## where R and n are as previously defined, Z and Z' are either H or ##STR3## and the --OH and ##STR4## terminal units of the bisphenol monochloroformate oligomers are present in an amount sufficient to provide an ##STR5## ratio having a value of from about 0.9 to about 1.1, and preferably from 0.9 to 1.0. The overall monochloroformate oligomer mixture comprises bischloroformate oligomers (both ends chloroformate), monochloroformate oligomers (one hydroxyl end and one chloroformate end), and bishydroxy oligomers (both ends hydroxyl). The relative proportions of these oligomers in the bisphenol monochloroformate polycarbonate oligomer are substantially 1:2:1 for the bischloroformate, monochloroformate, and bishydroxyl.
As is shown in copending application serial no. 519,980, the aforementioned bisphenol monochloroformate polycarbonate oligomers can be made by phosgenating a mixture of bisphenol under interfacial conditions, where the mixture contains sufficient alkali metal hydroxide to provide a pH of up to about 11.2. As the phosgenation continues, the pH of the mixture continues to drop until a pH set point, for example 8, is reached. Additional make-up aqueous alkali metal hydroxide can be introduced at various rates, until a signal is shown indicating bisphenol monochloroformate oligomer formation whereupon termination of base and phosgene introduction can be effected. If a particular control system is used, for example "Control System A", aqueous alkali metal hydroxide can be introduced into the phosgenation mixture at a rate which substantially maintains the pH of the mixture at the pH set point during phosgenation until a sudden rise in base demand occurs. Alternatively, a "Control System B" can be used which restricts the aqueous alkali metal hydroxide introduction to a rate sufficient to provide a ratio of the rate of mols of aqueous alkali metal hydroxide introduction to the rate of mols of phosgene introduction having a value of about 2.0 to about 2.5. With Control System B, the pH of the phosgenation mixture is found to cycle around the pH set point until it is found to substantially stabilize and thereafter suddenly falls to at least 1 pH unit below its previous stabilized value. These signals identifying either a change in pH or base flow into the phosgenation mixture establish the threshold point of bisphenol monochloroformate polycarbonate oligomer formation and the point at which phosgenation and base introduction can be terminated.
In copending application serial number 519,979, a method is described for making end capped polycarbonates and bisphenol monochloroformate polycarbonate oligomers. In U.S. Pat. No. 4,616,077, Silva, incorporated herein by reference, there is shown a method for preparing cyclic polycarbonate oligomer from bischloroformate. However, the Silva procedure for making cyclics is subject to excessive emulsion generation and erratic pH measurement and control.
It would be desirable to effect the cyclization of chloroformate intermediates which are not hydrolysis limited and which do not require excess water to prevent salt precipitation. In addition, it also would be desirable to minimize the production of the emulsions during the formation of such cyclic oligomeric polycarbonates to allow for a more accurate pH reading to avoid the production of degradation products during the formation of such cyclic oligomeric polycarbonates as well as minimize the production of the linear polycarbonates.